JP2015026452A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery superior in high-speed charge and discharge characteristics.SOLUTION: A lithium ion secondary battery comprises: a positive electrode 20 having a positive electrode active material; a negative electrode 10 having a negative electrode active material; a separator 30 disposed between the positive electrode 20 and the negative electrode 10; an electrode protection layer 40 disposed between the separator 30, and one or each of the positive electrode 20 and the negative electrode 10; an electrolytic solution; and a container 70 housing the positive electrode 20, the negative electrode 10, the separator 30, the electrode protection layer 40, and the electrolytic solution. Supposing that the total mass of N-methyl pyrrolidone in the container 70 is A, and the total mass of the negative electrode active material in the container 70 is B, A and B satisfy the following condition: 50 ppm≤A/B≤150 ppm.

Description

  The present invention relates to a lithium ion secondary battery.

  Conventionally, lithium ion secondary batteries in which an electrode protective layer is provided between an electrode and a separator are known. It is also known to use N-methylpyrrolidone when forming an electrode or an electrode protective layer.

JP 2006-48942 A

  However, the conventional method does not have sufficient high-speed charge / discharge characteristics. This invention is made | formed in view of the said subject, and it aims at providing the lithium ion secondary battery excellent in the high-speed charge / discharge characteristic.

The lithium ion secondary battery according to the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator disposed between the positive electrode and the negative electrode, the positive electrode and / or the negative electrode, and the separator. An electrode protective layer, an electrolytic solution, and a container for accommodating the positive electrode, the negative electrode, the separator, the electrode protective layer, and the electrolytic solution.
When the total mass of N-methylpyrrolidone in the container is A and the mass of the negative electrode active material is B, 50 ppm ≦ A / B ≦ 150 ppm.

  According to the present invention, the high-speed charge / discharge characteristics are excellent as compared with a conventional lithium ion secondary battery using N-methylpyrrolidone (NMP). Since the amount of residual NMP is small, it is considered that the film formed on the negative electrode surface due to N-methylpyrrolidone is reduced. This film becomes an electric resistance component and affects the charge / discharge characteristics of the lithium ion secondary battery.

  Moreover, it is preferable that the said N-methylpyrrolidone originates in the solvent in the slurry used for manufacture of the said electrode protective layer.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery excellent in the high-speed charge / discharge characteristic is provided.

FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to an embodiment of the present invention. FIG. 2 is a graph showing the relationship of 2s discharge power to A / B in the lithium ion secondary battery according to the example. FIG. 3 is a graph showing a relationship of 2s charging power with respect to A / B in the lithium ion secondary battery according to the example.

  An example of an embodiment according to the present invention will be described with reference to the drawings. As shown in FIG. 1, the lithium ion secondary battery 100 (power storage device) according to the present embodiment mainly includes an electrode assembly 50, a container 70, and leads 7 and 8 connected to the electrode assembly 50. Yes.

(Electrode assembly 50)
The electrode assembly 50 includes a plurality of negative electrodes 10, positive electrodes 20, separators 30, and electrode protection layers 40. The negative electrodes 10 and the positive electrodes 20 are alternately stacked with the separators 30 disposed between them. Moreover, the electrode protective layer 40 is arrange | positioned between the negative electrode 10 and the separator 30, and between the positive electrode 20 and the separator 30, respectively.

(Negative electrode 10)
The negative electrode 10 has a negative electrode metal foil 12 and a negative electrode active material layer 14 formed on both surfaces of the negative electrode metal foil 12. The negative electrode metal foil 12 has a tab portion 12a where the negative electrode active material layer 14 is not formed.

  The negative electrode metal foil 12 is made of a conductive material. An example of the material of the negative electrode metal foil 12 is a metal material such as stainless steel, titanium, nickel, aluminum, or copper, or a conductive resin. In particular, copper is suitable as the material for the negative electrode metal foil 12. Although the thickness of the negative electrode metal foil 12 is not specifically limited, For example, it can be set to 5-25 micrometers. Moreover, the thickness of the negative electrode active material layer 14 is not particularly limited.

  The negative electrode active material layer 14 includes a negative electrode active material and a binder, and may include a conductive additive as necessary.

Examples of the negative electrode active material are a carbonaceous material that can occlude and release lithium, an element that can be alloyed with lithium, an elemental compound that has an element that can be alloyed with lithium, or a polymer material.
Examples of the carbonaceous material are non-graphitizable carbon, artificial graphite, coke, graphite, glassy carbon, organic polymer compound fired body, carbon fiber, activated carbon, or carbon black. Here, the organic polymer compound fired body refers to a material obtained by firing and carbonizing a polymer material such as phenols and furans at an appropriate temperature.
Examples of elements that can be alloyed with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, 1n, Si, Ge. , Sn, Pb, Sb, Bi. Among them, the element that can be alloyed with lithium is preferably silicon (Si) or tin (Sn).
Examples of element compound having lithium can be alloyed _{elements, ZnLiAl, AlSb, SiB 4,} SiB 6, Mg 2 Si, Mg 2 Sn, Ni 2 Si, TiSi 2, MoSi 2, CoSi 2, NiSi 2, CaSi _{2, CrSi 2, Cu 5 Si} , FeSi 2, MnSi 2, NbSi 2, TaSi 2, VSi 2, WSi 2, ZnSi 2, SiC, Si 3 N4, Si 2 N 2 O, SiO v (0 <v ≦ 2 ), SnO _w (0 <w ≦ 2), SnSiO ₃ , LiSiO or LiSnO. An example of the elemental compound having an element capable of alloying with lithium is preferably a silicon compound or a tin compound. An example of the silicon compound is SiO _x (0.5 ≦ x ≦ 1.5). As the tin compound, for example, a tin alloy (Cu-Sn alloy, Co-Sn alloy, or the like) can be used. Examples of the polymer material are polyacetylene and polypyrrole.

  Examples of binders are fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, and styrene-butadiene rubber And synthetic rubbers such as carboxymethylcellulose. The amount of the binder can be 1 to 30 parts by mass with respect to 100 parts by mass of the active material.

  Examples of the conductive aid are carbon-based particles such as carbon black, graphite, acetylene black (AB), ketjen black (registered trademark) (KB), and vapor grown carbon fiber (VGCF). These can be added alone or in combination of two or more. Although it does not specifically limit about the usage-amount of a conductive support agent, For example, it can be set as 1-30 mass parts with respect to 100 mass parts active material.

  The negative electrode active material layer 14 prepares a slurry in which a negative electrode active material, a conductive auxiliary (if necessary), a binder, and a binder solvent are mixed, and is applied onto the negative electrode metal foil 12, and then the solvent is dried. Can be formed. Examples of the solvent are N-methylpyrrolidone, acetone, methyl ethyl ketone, cyclohexanone, methyl isobutyl ketone, ethyl acetate, butyl acetate, tetrahydrofuran and dioxane.

(Positive electrode 20)
The positive electrode 20 includes a positive electrode metal foil 22 and a positive electrode active material layer 24 formed on both surfaces of the positive electrode metal foil 22. At the end of the positive electrode metal foil 22, there is a tab portion 22 a where the positive electrode active material layer 24 is not formed. The positive metal foil 22 is made of a conductive material. An example of the material of the positive electrode metal foil 22 is a metal such as aluminum.

  The positive electrode active material layer 24 includes a positive electrode active material and a binder, and can include a conductive additive as necessary. Examples and blending amounts of the binder and the conductive additive can be the same as those described for the negative electrode 10.

  The positive electrode active material is not particularly limited as long as it is a positive electrode active material for a lithium secondary battery. An example of the positive electrode active material is a lithium compound. For example, lithium metal composite oxides such as lithium cobalt composite oxide, lithium nickel composite oxide, and lithium manganese composite oxide can be used. Other metal compounds or polymer materials can also be used as the positive electrode active material. Examples of other metal compounds include oxides such as titanium oxide, vanadium oxide, and manganese dioxide, or disulfides such as titanium sulfide and molybdenum sulfide. Examples of the polymer material include conductive polymers such as polyaniline and polythiophene.

In particular, the positive electrode active material is a lithium metal composite oxide represented by the general formula: LiCo _p Ni _q Mn _r O ₂ (p + q + r = 1, 0 <p ≦ 1, 0 ≦ q <1, 0 ≦ r <1). It is preferable to include. Examples of the composite metal oxide include LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.5 CO _0.2 Mn _0.3 O ₂ , LiCoO ₂ , LiNi _0.8 Co _0.2 O ₂ , and LiCoMnO ₂ can be used.
The positive electrode active material has a general formula: LiCo _p Ni _q Mn _r D _S O ₂ (p + q + r + s = 1, 0 <p ≦ 1, 0 ≦ q <1, 0 ≦ r <1, 0 <s <1) It is also preferable to contain the lithium metal complex oxide represented. D is at least one element selected from the group consisting of Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe, and Na.

  The positive electrode active material layer 24 prepares a slurry in which a positive electrode active material, a conductive additive (if necessary), a binder, and a binder solvent are mixed, and is applied onto the positive electrode metal foil 22 to dry the solvent. Can be formed. The example of a solvent is the same as that of manufacture of a negative electrode active material layer.

(Separator 30)
The separator 30 separates the positive electrode 20 and the negative electrode 10 and allows lithium ions to pass through while preventing a short circuit of current due to contact between both electrodes. An example of the separator 30 is a porous film made of synthetic resin such as polytetrafluoroethylene (PEFE), polypropylene (PP), polyethylene terephthalate (PET), or polyethylene (PE), or a porous film made of ceramics.

  As shown in FIG. 1, the end portion of the separator 30 preferably extends outward from the end surfaces of the positive electrode active material layer 24 and the negative electrode active material layer 14. A pair of separators 30 may form a bag so as to surround the negative electrode 10. The thickness of the separator 30 can be 10-50 micrometers.

(Electrode protective layer 40)
The electrode protective layer 40 is provided between the negative electrode 10 and the separator 30 and between the positive electrode 20 and the separator. The electrode protective layer 40 is a porous layer having ceramic particles and a binder that bonds the ceramic particles. Examples of ceramics are alumina, silica and titania. The particle size of the ceramic particles can be 0.2 to 2.0 μm. The kind of binder can be made the same as the binder mentioned by the negative electrode. The amount of the binder can be, for example, 1 to 30 parts by mass with respect to 100 parts by mass of the ceramic particles. The thickness of the electrode protective layer 40 can be 0.5-30 micrometers, for example.

  The electrode protective layer 40 may be fixed to the negative electrode 10 or the positive electrode 20 in advance, or may be fixed to the separator 30, and is not fixed to the positive electrode 20 and the negative electrode 10 and is not fixed to the separator 30. An aspect may be sufficient.

  The electrode protective layer 40 is prepared by preparing a slurry in which ceramic particles, a binder, and a solvent for the binder are mixed, and applying the slurry to the positive electrode 20, the negative electrode 10, the separator 30, or another base material, and drying the solvent. Can be formed. The example of a solvent is the same as that of manufacture of a negative electrode active material layer.

(Container 70)
The container 70 accommodates the electrode assembly 50 and an electrolytic solution (not shown). The material and form of the container 70 are not particularly limited, and various known materials such as resins and metals can be used.

(Electrolyte)
The electrolytic solution includes an electrolyte and a solvent that dissolves the electrolyte. The electrolytic solution is impregnated in the negative electrode active material layer 14, the separator 30, and the positive electrode active material layer 24.

Examples of the electrolyte are lithium salts such as LiBF ₄ , LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , and LiN (CF ₃ SO ₂ ) ₂ .

  Examples of the solvent are cyclic esters, chain esters, and ethers. Two or more of these solvents can be mixed. Examples of cyclic esters are ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma ptyrolactone, gamma valerolactone. Examples of the chain esters are methyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers are tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane.

  The density | concentration of the electrolyte in electrolyte solution can be 0.5-1.7 mol / L, for example. The electrolytic solution may contain a gelling agent.

(Lead)
The lead 7 is connected to the tab portion 12 a of each negative electrode metal foil 12. The lead 8 is connected to the tab portion 22 a of each positive metal foil 22. The ends of the leads 7 and 8 are outside the container 70.

(Concentration of N-methylpyrrolidone in the container)
In the lithium ion secondary battery according to this embodiment, when the total mass of N-methylpyrrolidone in the container 70 is A and the total mass of the negative electrode active material in the container 70 is B, 50 ppm ≦ A / B ≦ Satisfies 150 ppm. 75 ppm ≦ A / B, and 00 ppm ≦ A / B. Further, A / B ≦ 130 ppm can be satisfied.

  The N-methylpyrrolidone in the container 70 is derived from the solvent in the slurry used for manufacturing the negative electrode active material layer 14, the positive electrode active material layer 24, and / or the electrode protective layer 40. Most of the N-methylpyrrolidone is removed when the slurry is dried, but a part of the N-methylpyrrolidone remains in the layer and is contained in the container of the completed lithium ion secondary battery 100. In order to sufficiently reduce the concentration of N-methylpyrrolidone in the container, the drying temperature of the slurry containing N-methylpyrrolidone may be increased and / or the drying time may be increased. The drying temperature is preferably 100 ° C. or higher and preferably 125 ° C. or lower. The drying time is preferably 2 hours or more.

  The total mass of N-methylpyrrolidone in the container 70 can be measured by a gas chromatograph mass spectrometer (GCMS). For example, when measuring the concentration in the electrolytic solution, the electrolytic solution may be supplied to the GCMS as it is. When measuring the concentration in the solid matter such as the negative electrode, the positive electrode, the electrode protective layer, and the separator, After transferring the remaining N-methylpyrrolidone to acetone by applying ultrasonic waves, acetone may be supplied to GCMS.

(Function and effect)
According to the lithium ion secondary battery 100 according to the present embodiment, high-speed charge / discharge characteristics as compared with a conventional lithium ion secondary battery in which N-methylpyrrolidone is used as a solvent in the production of electrodes and / or electrode protective layers. Excellent. The reason is not clear, but it is considered that, for example, since the amount of residual N-methylpyrrolidone is small, a film formed on the negative electrode surface due to N-methylpyrrolidone is reduced.

The present invention is not limited to the above embodiment, and various modifications can be made.
For example, in the above embodiment, the electrode protection layer 40 is provided between both the negative electrode 10 and the separator 30 and between the negative electrode 10 and the separator 30, but electrode protection is provided only between either one. It can be implemented even if the layer 40 is provided.

Example 1
(Manufacture of negative electrode)
Graphite powder (negative electrode active material: particle size D50: 20 μm), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC) were mixed at a mass ratio of 98: 1: 1, respectively, and a solvent (ion exchange water) Was added to obtain a slurry. This slurry was formed on one side of a copper foil, the solvent was dried on a hot plate at 80 ° C. for 15 minutes, pressed, and further heated at 120 ° C. for 3 hours.

(Manufacture of positive electrode)
LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (particle diameter D50: 10 μm), acetylene black, and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 94: 3: 3, and the solvent (N-methylpyrrolidone) was added to obtain a slurry. This slurry was applied to one side of an aluminum foil, the solvent was dried on a hot plate at 80 ° C. for 30 minutes, pressed, and further heated at 120 ° C. for 3 hours.

(Formation of electrode protective layer (thickness 5 μm))
Alumina particles (D50: 0.5 μm) and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 96: 4, respectively, and a solvent (N-methylpyrrolidone) was added to obtain a slurry. This slurry is formed on both surfaces of the positive electrode and the negative electrode, and the solvent is dried on a hot plate at 80 ° C. for 15 minutes, pressed, and further heated at 120 ° C. for 3 hours to form an electrode protective layer on both surfaces of the positive electrode and the negative electrode Provided.

(Manufacture of batteries)
A polyethylene porous sheet was sandwiched between the positive electrode and the negative electrode to obtain a laminate. This laminated body was accommodated in a case in which both surfaces of an aluminum foil were laminated with a resin. Further, an electrolyte solution was supplied into the case, and then the case was sealed to obtain a plurality of lithium ion secondary batteries. The electrolytic solution contained a solvent and an electrolyte (1 mol / dm ³ LiPF ₆ ), and the solvent contained ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and fluoroethylene carbonate at a mass ratio of 30:30:40. . Moreover, the total mass B of the negative electrode active material in each battery was 84.9 mg.

(Example 2)
The drying conditions after pressing the positive electrode, the negative electrode, and the electrode protective layer were the same as in Example 1 except that the drying conditions were 120 ° C. and 6 hours. Moreover, the total mass B of the negative electrode active material in each battery was 84.1 mg.

(Comparative Example 1)
The drying conditions after pressing the positive electrode, the negative electrode, and the electrode protective layer were the same as in Example 1 except that the drying conditions were 100 ° C. and 3 hours. Moreover, the total mass B of the negative electrode active material in each battery was 84.4 mg.

(Concentration of N-methylpyrrolidone in the container)
The mass A of N-methylpyrrolidone in the battery container of each example and comparative example was measured by GCMS. In the order of Examples 1 and 2 and Comparative Example 1, A / B was 95 ppm, 131 ppm, and 403 ppm.

(Evaluation of high-speed charge / discharge characteristics)
The electric power per unit negative electrode active material mass that can be discharged at 25 ° C. for 2 seconds was measured for each battery having a state of charge (SOC) of 10%. The results of plotting the obtained power against A / B are shown in FIG.
For each battery having a state of charge (SOC) of 80%, the power per unit negative electrode active material mass that can be charged at 25 ° C. for 2 seconds was measured. The results of plotting the obtained power against A / B are shown in FIG.

  In the example satisfying 50 ppm ≦ A / B ≦ 150 ppm, it was confirmed that the rapid charge / discharge characteristics were excellent.

  DESCRIPTION OF SYMBOLS 10 ... Negative electrode, 20 ... Positive electrode, 30 ... Separator, 40 ... Electrode protective layer, 70 ... Container, 100 ... Lithium ion secondary battery.

Claims (2)

A positive electrode having a positive electrode active material,
A negative electrode having a negative electrode active material,
A separator disposed between the positive electrode and the negative electrode;
An electrode protective layer disposed between the positive electrode and / or the negative electrode and the separator;
Electrolyte, and
A container containing the positive electrode, the negative electrode, the separator, the electrode protective layer, and the electrolyte;
A lithium ion secondary battery satisfying 50 ppm ≦ A / B ≦ 150 ppm, where A is the total mass of N-methylpyrrolidone in the container and B is the total mass of the negative electrode active material in the container.   The lithium ion secondary battery according to claim 1, wherein the N-methylpyrrolidone is derived from a solvent in a slurry used for manufacturing the electrode protective layer.

Images